The present invention relates to a steerable chassis for a work vehicle.
A steering mechanism with two steerable wheels is generally used in order to change the direction of travel of a work vehicle or of any other kind of vehicle; in most cases, the steerable wheels are the front wheels, whereas in some cases, in some work vehicles such as fork-lift trucks, cranes, et cetera, the rear wheels are used.
For some work vehicles, especially vehicles that are not too large, such as small bulldozers, cranes, excavators, fork-lift trucks et cetera, it is advantageous to use a particular steerable chassis to change the travel direction of the vehicle instead of using a steering mechanism with steerable wheels.
These vehicles have a chassis that is substantially divided into two rigid portions that have no steerable wheels: a front chassis portion, which supports the front wheel axle, and a rear chassis portion, which supports the rear wheel axle.
The front and rear chassis portions are connected by means of an articulated coupling, which in a simple embodiment is substantially constituted by a hinge that has a vertical axis, for example of the fifth-wheel type.
Steering is achieved by acting on suitable actuation members that vary the angle formed by the longitudinal axes of the two chassis portions with respect to the fulcrum constituted by the hinge from a maximum value of 180°, for straight travel, to a minimum value, to which maximum steering curvature corresponds.
Substantially, during steering, the chassis of the vehicle literally “bends” at the hinge, thus allowing the operator to change the direction of travel of the vehicle.
Since this kind of vehicle usually lacks wheel suspensions, it is particularly uncomfortable for the operator, especially when traveling over bumpy roads.
In order to increase travel comfort, therefore, it has been thought to adopt, instead of the simple articulated coupling described above, a complex coupling that includes a hinge that has a substantially vertical axis, in order to allow the vehicle to steer, combined with an articulation whose axis is substantially parallel to the longitudinal axis of the vehicle.
A degree of freedom is thus added to the relative motion of the two chassis portions, because in addition to “bending”, the chassis portions are capable, by means of the articulation, of “oscillating” with respect to each other relative to an axis that is substantially parallel to the axis of the vehicle.
The axles of the front and rear wheels can thus assume a different inclination with respect to the ground, accordingly eliminating the torsional rigidity of the chassis and consequently increasing its travel comfort.
This solution is certainly advantageous for improving comfort of the vehicle during travel, but it can lead to drawbacks when the vehicle is instead stationary and is working, because it decreases its stability.
The work vehicles being considered are in fact used to lift loads of some kind by means of a suitable implement installed at the front of the vehicle, such as a fork lift in the case of a fork-lift truck, a bucket or an excavator arm in the case of an excavator, et cetera.
During lifting, if the center of gravity of the load accidentally falls outside the vertical, the oscillating motion of the two chassis portions about the articulation produced by the imbalance of weights may cause a destabilization of the vehicle that generates in certain cases a severe risk of lateral tipping of the vehicle.